1. Field of the Invention
The present invention relates generally to antennas for spacecraft and, more particularly, to a mounting means for individual radiating tiles within an antenna array.
2. Prior Art
The costs of communications spacecraft are under downward pressures due to competition among spacecraft manufacturers, and also due to competition with other forms of communications. Modularized spacecraft techniques are well known in the art. These techniques use standard modules to make spacecraft buses (payload carriers) of various sizes and capabilities, thereby reducing design costs, and particularly by reducing the need to space-qualify different structures which might be used to construct custom spacecraft using earlier techniques. Other techniques for reducing the costs of assembling buses have been implemented, such as misalignment tolerant fasteners.
Payloads have been more resistant to cost reduction, because they are, almost by definition, different from each other. Each spacecraft user specifies the number of communications channels which are to be carried, their frequencies, and the power to be delivered to a specified xe2x80x9cfootprintxe2x80x9d on the Earth""s surface. The electrical power modularization required to provide the desired total radio-frequency (RF) power is described in the prior art. The antennas, however, have been more resistant. In the past, reflector/feed antennas were used on the spacecraft, with the reflector and the feed being designed to provide the desired footprint over the specified frequency range. The reflector/feed arrangement using horn feed antennas exhibits high efficiency, which is very desirable in view of the electrical power limitations common to spacecraft. However, the reflector/feed antenna is difficult to design, and multiple feed horns may be required in order to provide the appropriate footprint.
Further, a reflector-type antenna is subject to physical distortion as a result of differential heating occasioned by insolation. The physical distortion, in turn, disrupts the desired footprint. Various RF-transparent insolation shields have been used to cover the radiating surface of reflector antennas, to minimize the distortion. To the extent that the thermal (or other) antenna distortion affects the footprint, no convenient remedy is available. When operation at a plurality of different frequency ranges is necessary, as when a satellite uplink and downlink are at different bands, such as C and X band, multiple reflector antennas are required, which exacerbates the abovementioned problems.
Further problems arise from the xe2x80x9cfrequency reusexe2x80x9d operating method, used to maximize the number of separate channels which may be used within each band, by transmitting alternate channels of each band with different polarizations, and using a polarization-sensitive reflector/feed arrangement, in that the reflector structure is much more complex than in a simple continuous reflector. The considerations relating to reflector/feed antennas have directed attention to other types of antennas for communications spacecraft, notably antenna arrays. Antenna arrays are well known in the art, and their use in conjunction with aircraft and spacecraft is well known, although the number of such arrays in actual use in spacecraft is very small, due to a number of practical problems. Among these problems is that of the size, weight, complexity, and the attenuation or loss of the beamformer, which is required to feed the RF signal to the antenna elements. Also, an array antenna must maintain a predetermined spacing between each antenna element and other elements of the array to prevent grating lobes.
Those skilled in the art know that antennas are reciprocal linear devices, in which the transducing characteristics during transmission and reception are the same. For example, the beamwidth, the gain (or more properly, the directive gain relative to an isotropic source) and the impedance at the feed points are the same in both transmitting and receiving modes. However, the terms used to describe antenna functions and characteristics were established at a time when this reciprocity was not apparent, and as a result the terms are suggestive of either transmission or reception, but generally not of both. Those skilled in the art know, therefore, that the description of an antenna may be couched in terms of either transmission or reception, or an intermixture of both, with the other mode of operation being understood therefrom. Thus, the term xe2x80x9cfeed port,xe2x80x9d for example, refers to the port to which signal energy is applied during transmission, and is also applied to that same port at which signal energy is received in a receiving mode.
Array antennas are of two general types, active and passive. The xe2x80x9cactivexe2x80x9d antenna array includes active devices such as semiconductor devices to aid in reception or transmission, or both; a passive antenna array does not. The proper phase characteristics between the elements of the array must be provided in some way in either the active or passive arrays. An active antenna array will generally include controllable phase-shifters which can be used to adjust the phase of the RF signal being fed to one (or to a subset) of the antenna elements of the array. The need for a phase-shifting beamformer may be avoided by using a non-phase-controlled signal amplitude divider, in conjunction with control of the phase control elements associated with each element or subset of elements. An active antenna array will often have a transmit amplifier and a receive amplifier associated with each antenna or subset of antennas. These amplifiers add to the cost and complexity of the system, and are a major source of waste heat, which adds to the insolation heat, and must be taken into account. The cumulative effect of the heat absorbed by the array antenna, and that generated within the array antenna, tends to raise the temperature gradient of the array antenna, and to cause physical distortion, which in turn affects the radiation pattern and the resulting footprint. In general, antenna arrays for use in spacecraft have to address requirements to minimize weight, RF signal losses, and, in active embodiments, the energization power, as well as to satisfy waste heat removal requirements. The advantages of array antennas include the ability to control the beam characteristics by remote control of the phase shifters. Also, an array antenna may be folded for launch and then deployed or erected.
U.S. Pat. No. 5,666,128 to Murray et al. proposes the use of flexible beams by which each rigid tile, in an array of tiles, is attached to an antenna frame. The flexible beams via their bending properties, allow for expansion and contraction of the tile with respect to the frame thus preventing accumulation of tile distortions across the antenna array. However, the flexible beams do not tension the tile, instead they allow the rigid and relaxed tile to augment the otherwise inadequate lateral shear properties of the antenna frame via their tensile and compressive properties.
In view of the prior art, there is a need for an improved spacecraft antenna structure that bias an array of flexible membrane elements diagonally in tension such as to augment the otherwise inadequate lateral shear properties of the antenna frame.
Accordingly, an array antenna is provided. The array antenna comprises:
a frame having a two-dimensional array of a plurality of openings; an
electromagnetically radiating tile disposed in each opening; and mounting means for holding at least one tile in a corresponding opening of the frame, each of the mounting means comprising at least two biasing members, each biasing member exerting a biasing force on the tile relative to the frame.
Preferably, each of the radiating tiles includes an array of radiating elements and the frame lies essentially in a plane, and the plane of each tile is parallel to the plane of the frame.
In a preferred implementation of the array antenna of the present invention, each biasing member comprises a leaf spring having first and second ends attached to the frame and a bowed section. In yet another preferred implementation of the array antenna of the present invention each biasing member further comprises means for varying the biasing force in response to a change in temperature. When the biasing members are leaf springs, the means for varying the biasing force in response to a change in temperature preferably comprises the bow portion being fabricated from a first and second material, each having a different coefficient of thermal expansion.
In a second preferred implementation of the array antenna of the present invention, the mounting means comprises a radial tensioning wire fastened to each of four corresponding comers of the tile at a first end and slipably disposed to a tensioning yoke at a second end, the tensioning yoke having a tensioning member for maintaining the radial wires and tile in tension.